Stabilization and Extraction of Fluoride Anion Using a Tetralactam Receptor

A neutral tetralactam macrocycle was prepared in a few minutes in one pot and at high concentration using commercially available starting materials. NMR titration studies in DMSO revealed an anion affinity order of F^- > AcO^- > Cl^- > Br^-. The receptor affinity for F^- is very high due in part to formation of a self-complementary dimer comprised of two "saddle shaped" complexes. An X-ray crystal structure showed that the two F^- ions within the dimer are separated by 3.39 Å. The electrostatic penalty for this close proximity is compensated by attractive interactions provided by the surrounding tetralactam molecules. Reactivity experiments showed that stabilization of F^- as a supramolecular complex abrogated its capacity to induce elimination and substitution chemistry. This finding raises the idea of using tetralactam macrocycles to stabilize fluoride-containing liquid electrolytes within redox devices such as room-temperature fluoride-ion batteries. A lipophilic version of the tetralactam macrocycle was prepared and used to extract F^- from water into a chloroform layer with high efficiency. The favorable extraction is due to the architecture of the extracted dimeric complex, with all the polarity located within the core of the self-associated dimer and all the nonpolar functionality on the exterior surface.